This invention relates to an ignition timing control apparatus for an internal combustion engine, and in particular to an ignition timing control apparatus for an internal combustion engine which is operable either with a low-octane rating fuel or a high-octane rating fuel, or with a mixture thereof.
It is well known in the art that the octane rating of fuel gasoline is greatly interrelated with a knock-resistibility in an internal combustion engine. Namely, as the octane rating of gasoline becomes higher, the knocking of an engine correspondingly decreases.
FIG. 1 shows an ignition timing vs output shaft torque characteristic in an internal combustion engine in the case where a commercially available low-octane gasoline (regular gasoline) and a commercially available high octane gasoline (premium gasoline) of which the octane rating is higher than that of low-octane gasoline. In the figure, point A indicates a knock limit for low-octane gasoline, and point B indicates a knock limit for high-octane gasoline, in which knock arises when the angle of the ignition timing is advanced beyond these points.
According to FIG. 1, it is possible to advance the angle of the ignition timing up to a point B when high-octane gasoline is used, where the output shaft torque will be higher as compared with the use of low-octane gasoline.
Thus, in an engine associated with such a characteristic, in the case where regular and premium gasolines are alternatively used and the ignition timing of the engine is changed over to have an optimum value according to the kind of gasoline in use, or in the case where a mixture of both gasolines is used and the ignition timing is advanced corresponding to the mixing ratio of regular and premium gasolines, the output of the engine can be enhanced.
However, in a conventional control apparatus for an internal combustion engine, the reference ignition timing characteristic is preset only for a predetermined octane rating gasoline, e.g. the low-octane rating regular gasoline. Therefore, when gasoline of the engine is changed over from low octane rating gasoline to high octane rating gasoline or when a mixture of both gasolines is used, an increase in the output of the engine can not be expected without any change or modification thereof, so that the presetting of the reference ignition timing must be repeated towards an advance angle, by some method.
Particularly, such a repeated presetting of the reference ignition timing is very difficult to achieve when using a mixture of both gasolines because, as shown in FIG. 1, the knock limit moves between curves A and B in relation to the mixture ratio and, accordingly, the advance angle limit is variable.
Further, at the initial operation of the engine, intensive knocks may disadvantageously arise depending on the initial setting of the ignition timing of the engine.
U.S. Pat. No. 3,822,583, titled "Method for Determining Octane Rating of Fuels under Road Conditions" discloses measuring the octane rating of a fuel being used for an engine by the detection of knocks.
Japanese Patent Application Laid-open No. 58-57072, published on Apr. 5, 1983, titled "Method for Controlling Ignition Timing of Electronic Controlled Engine" discloses a change-over ignition timing control apparatus for an engine depending on the octane rating of a fuel being used.